- Make sure the theme path of 'page_not_found' is set in site_theme_paths.php
- Go to the 404-page: Sitemap -> Check 'Show system pages' -> Page not found
- Add the 'Search suggestions' block
- Select your search destination page (e.g. /search/)
- Select the name of the template you'd like to use for the search results. I've included a modified 'view.php' file from the search block in the folder /useful. You could copy this file to /blocks/search/templates/

I don't get search suggestions

- Flush the cache
- Be sure to disable full page caching for 404-page
- Try other URL's. /this-url-doesnt-exist/
- Enable the debug option

I still don't get search suggestions! 

- This is probably due to a bug in the search controller. Therefore I've included a file named 'controller.php' in the folder 'useful'. Copy this file to /blocks/search/controller.php and try again.

'Disable output'

If you check this option, no results will be shown. It only adds search keywords to the $_REQUEST['query'] variable, which is used by the search block.